The Magnetic Evolution of Binary White Dwarf Merger Remnants

Master Thesis Defense for Suoqing Ji
Advisor: Dr. Robert Fisher
Committee members: Dr. David Kagan, Dr. Gaurav Khanna & Dr. Robert Fisher
Title: The Magnetic Evolution of Binary White Dwarf Merger Remnants
Abstract: Type Ia supernovae result from the thermonuclear explosion of a carbon-oxygen white dwarf star whose mass is close to the Chandrasekhar limit -- roughly 1.4 times the mass of our sun. The brightnesses of typical Type Ia supernovae explosions are highly similar, providing us with a standard candle with which we can accurately measure space and time in the cosmos. Using Type Ia supernovae, astronomers have accurately measured the rate at which space itself is expanding, and have inferred the existence of a mysterious new type of energy -- dark energy. However, the nature of the progenitors which give rise to Type Ia supernovae, and of the explosion mechanism itself, remain poorly understood. Consequently, an improved understanding of the the nature of Type Ia supernovae and their progenitors is central to unlocking the mystery of dark energy.
In this research, we focus upon the double-degenerate scenario, in which merging binary white dwarf systems give rise to Type Ia supernovae. In these systems, the merging white dwarf binary system produce a rapidly-spinning white dwarf merger surrounded by a hot disk. We perform the first multidimensional simulations of these white dwarf merger remnants to include a full treatment of magnetohydrodynamics. We demonstrate that the disk surrounding the white dwarf merger is highly-unstable to an initially-weak magnetic field. In the differentially-rotating disk, the magnetic field is significantly amplified as fluid elements are destabilized by the Lorentz force. Our simulations further demonstrate, for the first time, that this process, referred to as the magnetorotational instability, produces a high-field magnetic white dwarf, with a surface field exceeding 10^8 G. Furthermore, we find that two effects -- compression driven by mass accretion powered by the magnetorotational instability, and the spin-down of the merger caused by magnetic braking -- significantly increase the central peak density and temperature of the merger, leading to a central nuclear ignition. This sequence of events provides a possible approach to the origin of Type Ia supernovae for some double-degenerate mergers.